1. Field of the Invention
The present invention relates generally to a circuit for detecting electromagnetic radiation and, in particular, a circuit for detecting electromagnetic radiation having at least one sensor element which converts the radiation into an electrical signal and having a field effect transistor receiving the electrical signal.
2. Description of the Related Art
Detectors are known for detecting electromagnetic radiation such as heat radiation or infrared radiation. One application for such detectors is as motion detectors. Motion detectors detect, for example, persons by detecting the heat radiation emitted by the persons. After detection of a person, the motion detector emits a signal which can be further processed as desired. For example, a door opener may be actuated, a light may be switched on, or an alarm may be triggered upon the detection of a person within the sensing field of the motion detector. The signal which is being sought for use in these further processes, which is infrared radiation from the person being detected, generally changes at a low frequency. An important region for a signal detection is around 1 Hz. The frequency of the signal results from the speed with which the infrared source, the person, passes by the sensor elements.
A known sensing circuit is shown in FIG. 1 as an example of one design. A sensor which operates capacitively serves as the sensor element. The sensor element of the illustrated exemplary circuit is a pyroelectric cell 2. The sensor cell produces charges corresponding to a change in the intensity of the infrared radiation striking it, and stores these charges capacitively. One terminal of the sensor element is connected to a fixed reference potential, such as ground. The other terminal of the sensor 2 supplies a voltage as an output signal. Since the output has an extremely high impedance, an impedance converter is connected to the output so that the evaluation circuit which is connected for utilizing the sensor output sees a sufficiently low equivalent resistance for the circuit 1. A high impedance resistor 4 is connected in parallel to the sensor element 2. The resistor 4 ensures that charges which are accumulated in the capacitive sensor 2 are eventually discharged so that the charge disappears after the heat source which triggered the charge has disappeared. The impedance converter in the standard circuit is a field-effect transistor 3. One terminal of the field-effect transistor 3 is connected to a supply voltage U.sub.B while the other terminal of the field-effect transistor 3 supplies the output signal U.sub.A for further processing. In a circuit using an n-channel junction gate field-effect transistor (FET) 3, the sensor element 2 is connected between ground and the gate of the FET, the drain is connected to the supply voltage U.sub.B and the source is connected to supply the output signal U.sub.A.
The sensor elements 2 have an extremely high characteristic impedance which is on the order of magnitude of 100 G ohms. As a result, the output signal of the sensor elements is very weak, rendering the entire circuit unusually susceptible to electrical disturbances. Radio frequency electrical disturbances are mainly the cause of problems in the sensor circuit For the present invention, radio frequency refers to frequencies in the MHZ and GHz range. Radio frequencies which are coupled in via the supply lines are particularly disturbing to the circuit operation. However, directly received radio frequency disturbances also play a part, such as those from radio telephone devices or the like. The radio frequency disturbances lead to false detections and, thus, malfunctioning of the circuit 1.
In an effort to reduce such false detections, a capacitor 5 has been used, which is connected between the signal output 7 and ground. The capacitor functions as a low pass filter which short circuits the high output frequencies so that these high frequencies are attenuated at the output of the circuit.